A method of this kind is known, for example, from U.S. patent application publication U.S. 2004/0055561 A1 wherein a method is described for heating a catalytic converter in internal combustion engines having gasoline-direct injection with the steps:
shifting the ignition to “retard”;
checking whether the charge of the cylinders with air exceeds a pregiven threshold;
subdividing the fuel injection into two component quantities which are injected before the ignition when the air charge exceeds the threshold.
Vehicles having internal combustion engines require catalytic converters in the exhaust-gas system for exhaust-gas purification. These catalytic converters must be brought to the operating temperature as fast as possible after a cold start so that means for heating are provided. For example, after a cold start, the catalytic converter can be heated via high exhaust-gas temperatures. This so-called motoric catalytic converter heating has the advantage that it can be done without additional components.
In internal combustion engines, the exhaust-gas temperature can, in principle, be increased in that the degree of efficiency of combustion is deteriorated. A deterioration of the degree of efficiency of the motoric combustion can, for example, be brought about by a deviation of the ignition time point from the optimal time point. The optimal time point is defined by the maximum degree of efficiency. With a reduction of the degree of efficiency, the exhaust gas is hotter compared to the operation without a deterioration in the degree of efficiency. Accordingly, the exhaust gas develops an intensified heating action in the catalytic converter.
For engines having gasoline-direct injection, there exist, in principle, two possibilities to increase the exhaust-gas temperature without adding additional components:
1. Retarded ignition to deteriorate the degree of efficiency of combustion. The ignited mixture is stoichiometric or slightly lean.
2. Additional injection of fuel after ignition for follow-on combustion. The ignited mixture is very lean (stratified operation).
With the increasing rough running, the retarded ignition is limited for a homogeneous mixture. The emissions can furthermore be improved by a slightly lean exhaust-gas lambda at low catalytic converter temperatures. A leaning is, however, only possible to a limited extent for a cold engine.
If a secondary injection is provided for the catalytic converter heating, then the complete combustion of the additional fuel mass has to be ensured. In order to ensure a reliable and complete combustion in the exhaust manifold, the latter must be optimized in its configuration with respect to through mixing and low thermal mass. Other targets, such as the reduction of structural space and the optimization of power can thereby be limited. In principle, the after-reaction takes place to a poorer degree in a cold exhaust manifold. Accordingly, the emissions can hardly be reduced shortly after the start.
Because higher temperatures are present in the combustion chamber, low emissions can be achieved already shortly after start with an after burning in the combustion chamber. If the fuel is still to be ignited in the combustion chamber, then the operating parameters must be held within a narrow window. Especially, the injection must start very early and therefore contributes significantly to the torque development. Very short injection times are a precondition for small load points which implies very high demands on the injection valves.
The mixture preparation changes with the subdivision or splitting up of the injection in advance of ignition. With this type of mixture, the engine running can be improved. Basically, for a poorer degree of efficiency and therefore a retarded ignition time point and a higher exhaust-gas temperature, an improved rough running is achievable and the mixture can be more greatly leaned earlier after the start than for a homogeneous mixture by simple injection. In this way, lower emissions arise.
However, the accuracy of injection valves at small quantities is very poor. For this reason, a subdividing of the injection for smaller air charges is not possible.
In order to ensure a reliable start and run-up of the engine, that is, of the internal combustion engine, a simple homogeneous injection can still be necessary for this phase. A subdivision of the injection takes place only when there is a sufficient air charge. In this way, short injection times are avoided which would lead to an imprecise fuel metering.
Because of the subdivision of the injection, a mixture stratification arises. In this way, a rather rich mixture can be present at the spark plug while the lambda sum is still lean. A reliable ignition even for a lean lambda sum is ensured by the rich mixture about the spark plug.
Notwithstanding late ignition, a reliable rapid ignition of the mixture can additionally be ensured whereby the quiet running with late ignition is improved.
A different mixture distribution adjusts with divided injection taking place in advance of ignition, namely, rich in the center of the combustion chamber and lean on the wall of the combustion chamber. For this reason, the wall heat loss can be reduced. Depending upon the combustion chamber form and the parameters, the following effects can result:    (i) a higher exhaust-gas temperature with the same exhaust-gas quantity and therefore more heating power for the catalytic converter;    (ii) a low exhaust-gas quantity at the same temperature because the wall heat losses are lower whereby the dwell times of the toxic substance components in the exhaust manifold and in the catalytic converter become longer and an after-reaction is required. The emissions after the catalytic converter can therefore also be improved hereby.
Basically, at least once in the start phase, there must be a switchover from the simple homogeneous injection (start and run-up) to the divided injection (heat up of the catalytic converter) and back. Since the ratio for the injection quantity cannot be varied or can be varied only slightly, there must be a hard switchover between these two types of mixtures.
Here, it can happen that the driver perceives the switchover, which can include an abrupt change in torque, as a jolt in the motor vehicle. The torque development is very different for simple homogeneous injection and subdivided injection because of different mixture types and the different combustion speeds. For this reason, the ignition time point must be abruptly shifted with the switchover and the air charge must be rapidly changed. Even when the torque development for this change is precisely modulated, inaccuracies result because of tolerances of sensors and actuators, for example, via the imprecise detection of the air charge and the crankshaft angle.
Furthermore, inaccuracies of the fuel injection are also present because two short injection times compared to one long injection time are given. In this way, lambda deviations can additionally occur. This problem can be eliminated only by more accurate injection valves.